1. Field of the Invention
The present invention relates to an endoscope and an endoscope apparatus, and in particular, to an electronic endoscope equipped with a fixed focus image pickup unit constructed of an objective optical system and a solid state image pickup device.
2. Description of the Related Art
In an endoscope which is equipped with a so-called fixed focus image pickup unit in which a solid state image pickup device is fixed to an image position of an objective optical system and there is no movable part, it is possible to simplify and lessen the structure of the image pickup unit, and thus it is advantageous in respect of cost reduction, lessening of an outer diameter of an endoscope insertion section, and shortening length of a rigid end section. In addition, since there is no need for focusing, a user can concentrate on the operation of an endoscope and easily treat it, and hence, it is used widely in various fields such as medical use and industrial use.
In an endoscope equipped with such a fixed focus image pickup unit, a focus position where a clear image can be obtained also in a distant view (for example, about 50 to 100 mm in the case of a common medical use endoscope) is set so as to perform an operation of guiding an endoscope insertion section end to a position which it is intended to observe, and a so-called screening that selects a desired observation position with watching a large area. In addition, it is common that, when a distance between an endoscope insertion section end and an object is shortened, an object distance in which a clear image is obtained is up to about 5 to 10 mm.
When it is required to observe an object in detail enlargingly, it is conceivable to provide and construct a so-called zoom image pickup unit which enables a zooming operation (zooming) of moving a lens in an objective optical system to change a focal length and an operation distance. As to endoscopes equipped with such a zoom image pickup unit, various proposals such as Japanese Patent Laid-Open No. 2000-330015, and Japanese Patent Laid-Open No. 2001-33710 have been made up to now.
In an endoscope equipped with a zoom image pickup unit, it is possible to obtain almost the same depth of field as that of an endoscope equipped with a fixed focus image pickup unit at a so-called wide end at which a focal length becomes short. Therefore, in this case, usage similar to that of an endoscope equipped with a fixed focus image pickup unit can be performed. In addition, since a depth of focus approaches toward a near point (becomes shallow) at a so-called tele end where a focal length becomes long in comparison with the wide end, it becomes out of focus at a distant view (about 50 to 100 mm). Instead, it is possible to obtain a clear image when a distance between an endoscope insertion section end and an object approaches (for example, from about 2 mm to 3 mm in the case of an endoscope equipped with a general zoom image pickup unit). Hence, it is possible to enlargingly observe an object in detail.